The present invention relates to data displays, and more specifically concerns apparatus and methods for increasing the legibility of characters having different display attributes or modes.
Many present data-processing displays allow alphanumeric character images to be presented with different display attributes, such as highlighting, blinking, and normal/reverse. A "display attribute" is a visible aspect of a character image which exists independently of the identity of the character. For the normal/reverse attribute, images are selectively displayed as either bright characters on a dark background (normal mode) or as dark characters on a bright background (reverse mode).
Conventional practice with respect to the normal/reverse attribute, for both cathode-ray tube (CRT) and other displays, merely inverts a binary video signal to switch between normal and reverse modes. If a binary "1" represents a bright dot and a "0" represents a dark space, then the ones and zeros of the video signal for the normal mode are merely inverted by a simple logic circuit to obtain the reverse-mode video signal.
Mere inversion of the video signal makes the reverse-mode characters more difficult to read. In CRT displays, the problem is compounded because the CRT itself makes dark dots appear less wide than bright dots having the same time duration. This problem has been solved by time-stretching the (dark) character-image dots in reverse mode more than the (bright) character-image dots of the normal mode, as described in copending commonly assigned U.S. Pat. No. 4,555,701, patented Nov. 26, 1985 by R. E. Dahl et al.
The legibility problem, however, goes deeper than merely making reverse-mode characters appear the same as the corresponding normal-mode characters. Even when the physical properties of the display are compensated for, it has been found that a character font which is highly legible in normal mode is less easily legible in reverse mode, and vice versa. In the prior art, therefore, a designer had to optimize a character font for one mode--usually normal mode--and accept less readability in the other mode, or compromise the font design and detract from both modes.
In an environment where ever larger numbers of people spend increasing amounts of time at data displays, especially for long continuous intervals, even apparently small changes in font design can greatly enhance legibility. This in turn can reap great rewards of operator comfort, happiness, and even health.